How to carry out a Percolation Test for a Soakaway?

What is a percolation test?

A percolation test is the last of three mandatory tests used to determine if a new or replacement soakaway can be installed. You will first need to undertake a Groundwater Source Protection Zone search and a Trial Site Assessment Hole to assess the water table in your area fully. 

The percolation test measures how quickly water drains away from the soil. Different soil types will produce a variety of results. For example, fine sandy soil will drain water away significantly faster than thick clay soil. 

The testing process is fairly simple. It involves digging a pit to specific dimensions, filling it with water, and then timing how long the water takes to soak into the soil. 

What will the results of the percolation test determine?

The results of the percolation test will determine how large your soakaway will need to be as well as if you can even install one. The results of this test can also be used to determine the required size of the drainage field.

The results can also be used to demonstrate to your local authority that you have carried out the test correctly. To support this we advise you take time-stamped photos of each of the steps, as described below.

What are soakaways?

There are two general types of soakaway:

  • Foul water soakaway – These are used when septic tanks and sewage treatment plants are in place. This must be done using perforated BS-EN1401 underground sewer pipe, bedded on gravel (20mm clean gravel is usually used).

  • Surface water soakaway – These have to handle larger volumes of water in a short period of time. Soakaway crates are the favourable option, compared to a hole filled with rubble This method is not allowed for septic tanks and sewage treatment plant soakways. They are only for rainwater and surface water soakaways.

A relatively large amount of space is required for a soakaway to be considered. The soakaway must be a minimum of 5m away from your building, 5m away from a road and 2.5m away from any boundary.

What are the steps of the percolation test?

Here are the steps required for undertaking a percolation test for foul water soakaways:

  1. Excavate a hole – Dig a hole 300mm square to a depth of at least 300mm below the proposed invert, spacing them along the proposed line of the where the soakaway would be. While digging the hole, look out for changes in soil characteristics at different depths.
  2. Saturate the local soil – Fill each hole with a depth of at least 300mm.
  3. If the water drains rapidly – If it drains within 10 minutes, the hole should be refilled up to 10 times. If the water continues to drain away then this soil is unsuitable for a soakaway. And if it hasn’t soaked away within 6 hours it is also not suitable.
  4. Determine the percolation rate – Refill the test holes with water to a depth of at least 300mm and then using a stopwatch see the time, in seconds, for the water to seep away from 75% to 25% full level. 
  5. Divide this time – Divide the time by 150. The answer gives the average time in seconds (V) required for the water to drop 1mm.
  6. Repeat this test – Repeat the test 3 times for each hole at different times in the day. The average figure from the tests should be taken by adding all the results together and then divided by the number of holes, giving us the Vp.
  7. If results vary – Where the Vp results vary widely, make further tests on a minimum of three different locations in the proposed area for a soakaway.

If the test results show an average Vp value of between 15 and 100 then the ground is suitable for a soakaway. The minimum of 15 ensures that untreated effluent cannot percolate too rapidly into the ground resulting in polluting the groundwater. If the Vp is above 100, effective treatment is unlikely due to inefficient soakage leading to wastewater pounding on the land surface. 

Please note:
When performing a percolation test the weather conditions must be right. Severe or abnormal weather conditions such as heavy rain or frozen ground or a draught will lead to inaccurate results due to the changed water table.


How to perform the percolation test calculation?

Sewage treatment plants: Area (A) = V X P X 0.20 

Septic tanks: Area (A) = V X P X 0.25

V = The amount of time in seconds for the water in the test hole to drop by 1mm
P = The maximum number of persons that the tank or plant is designed to serve